Chapter 7 Lab Exercise: Mode Conversion Due to Changes in Cable-to-Board Electrical Balance

spectrum analyzer with tracking generator connected to a current probe around a twisted pair wire

Emissions and immunity problems are often the result of mode conversion in a signal path. To avoid mode conversion, it's necessary to maintain the same level of electrical balance from the source through the transmission line to the load. 

Preparation: Students need to become familiar with the definitions of differential-mode and common-mode as described in Chapter 7.

Equipment Required:

  • vector network analyzer or (spectrum analyzer with tracking generator)
  • a coaxial cable (30-100 cm)
  • a twisted or untwisted wire pair cable (same length as coax)
  • RF current probe (30-100 MHz)
  • balun or isolation transformer that works up to 100 MHz
  • circuit board with two 50-Ω microstrip traces running parallel to each other
  • 50-Ω board-edge terminations
  • snap-on ferrite cores (optional)

Procedure:

Step 1: Connect Port 1 of the VNA to one section of the coaxial cable. Connect the other end of the cable to one of the 50-Ω microstrip traces on the circuit board. Terminate the other end of the trace with a matched (50-Ω) termination resistance.

Step 2: Connect Port 2 to the RF current probe and place the probe around the coaxial cable at a fixed and repeatable position. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (This will be a very small value, perhaps below the noise floor. Reducing the resolution bandwidth and video bandwidth may lower the noise floor sufficiently to detect this current.)

Step 3: Disconnect the coaxial cable from the board and reconnect it so that each conductor in the coax connects to a different microstrip trace. Short the opposite end of the trace connected to the coaxial cable shield. In this configuration, the signal return current flows on a trace rather than the board's return plane. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (The imbalance change at the connection between the cable and board creates a common-mode voltage that drives the cable relative to the board. A significant common-mode current should be detected.)

Step 4: Place the balun on the Port 1 output of the VNA and use it to drive the wire-pair cable. Connect the other end of the cable to the two microstrip traces. Terminate each trace with a 50-Ω resistance to the board's plane. 

Step 5: Connect Port 2 to the RF current probe and place the probe around the wire-pair cable at a fixed and repeatable position. Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (This will be a small value.)

Step 6: Disconnect one of the wires at the board and connect it to the board's ground plane instead.  Record the peak amplitude and frequency of the measured common-mode current between 30 and 100 MHz. (The imbalance change at the connection creates a common-mode voltage that drives the wire pair relative to the board. A significant common-mode current should be detected.)

Step 7: Try clamping a snap-on ferrite core around the cable at various positions. How much reduction in the common-mode current can be achieved this way?

Notes: 

The common-mode current is proportional to the differential-mode voltage at the connection as well as the change in the imbalance. Both the balun and the different termination impedances alter this voltage between test set-ups. This voltage difference is easily calculated and can be accounted for when comparing the common-mode currents in the different test configurations.